Liquid crystal materials are well known, and are commonly used in electro-optical devices such as watches, calculators, displays etc. Such materials are of two general types. There are those which use the electro-optical properties of the nematic (N) phase, such as the electrically controlled birefringence (ECB) effect, as described in M. F. Schieckel and K. Fahrenshon, "Defomation of nematic liquid crystals with vertical orientation in electrical fields". Appl Phys Left (1971), 19, 3912. There are also those which use the electrooptical properties manifested by smectic phases. Examples of the latter include the ferroelectric effect manifested by certain chiral tilted smectic phases, see for example N A Clark and S T Lagerwall Appl-Phys Left (1980) 36, 899 which offers the advantages of high speed and bistable properties. The chiral smectic C, F and I phases (S.sub.C * S.sub.F * and S.sub.I I (the asterisk denoting chirality) are generally most favoured for such user the S.sub.C * being preferred because of its lower viscosity. Another electro-optical effect in smectic phases is the fast-switching electroclinic effect manifested by smectic A phases (S.sub.A).
Nematic and ferroelectric smectic liquid crystal materials have a number of desirable requirements in common, which for example include ease of preparation, chemical and photochemical stability, low viscosity and a broad temperature range over which the useful phases (eg N, S.sub.C, S.sub.F or S.sub.I) persist. Such materials are generally mixtures of compounds and these requirements are sought in compounds or mixtures thereof intended for use in the materials.
Materials and compounds for use in ECB devices and S.sub.C * devices have a number of specific desirable requirements. For example for use in ECB devices, compounds and materials should preferably show a high value for the ratio of elastic constants K.sub.3 /K.sub.1, high values for the optical anisotropy .DELTA.n and a negative dielectric anisotropy .DELTA.E. Indevices which use the ferroelectric effect in S.sub.C * materials it is desirable that compounds and mixtures show an S.sub.A phase at a temperature above the range over which the S.sub.C * phase persists, and that undesirable phases such as S.sub.B do not appear.
A number of compounds are known which show broad S.sub.C phases, among which are the alkyl- or alkoxy- terminated fluoroterphenyls ("FTP's") described in EP-A-0132377, of general formula: ##STR1## where R.sub.1 and R.sub.2 are alkyl or alkoxy. Some FTPs and mixtures thereof are known to show broad temperature range S.sub.C phases. This has made them excellent candidates for constituents of ferroelectric smectic mixtures. See for example their use in the mixtures disclosed in PCT/GB 87/0441 and GB 8627107.
It is an object of the present invention to provide novel compounds which may be used as constituents of liquid crystal mixtures, and novel liquid crystal mixtures which incorporate them.
According to this invention novel laterally cyano-substituted terphenyls of general formula I below are provided: ##STR2## wherein R.sup.1 and R.sup.2 are independently selected from hydrogen or C.sub.1-15 alkyl, alkoxy, or alkyl or alkoxy in which one or more CH.sub.2 groups are replaced by O, COO, OOC, CHX, CX.sub.2, CH=CX, CX=CX, where X ia Fluorine or chlorine, CRCN where R is alkyl, or C.tbd.C, or in which X is terminal. CH.sub.3 of the said alkyl or alkoxy chain is replaced by CF.sub.3, n is 0 or 1, and the CN and F (if present) substituent are independent located in any of the available substitution positions.